Will my thin horizontal beam not break down the middle under a heavy load?

I didn't have enough thick logs for the front of my house, so I used the thickest I had, a ~12cm (4.75in) spruce, and last fall I made the joint. I recently disconnected the mortise because I forgot to drill peg holes last season and I see the wood has split down the middle a considerable bit (see the photos). I'm worried it could crack in half with enough weight, especially as it supports the roof. Will an oak peg suffice to keep the log together no matter the load? or should I bite the bullet and redo the horizontal beam with something thicker? The roof will be on a 55º angle and can I get away with pegging it together and inserting a vertical beam down the center and two wind braces to make up for the thin beam?

I would replace it .  

Hi Maruf;
I'm only a part time wood worker so my opinion is of limited value at best.

Your right, that joint looks a bit flimsy but, its job is not to support weight but to stabilize the structure.
Once seated that joint will bear weight over the size of your vertical post.  
Adding two diagonal braces from the vertical up to the horizontal will strengthen and stabilize the roof.
You can always put a steel band around the cracking part of the log if it concerns you at a later date.

55 degree roof is an A frame structure  you will not have any snow buildup up high at all  so the only weight up there will be the roofing materials and the roofers while installing.

It is always best to over build a structure but sometimes you work with what you have.

In a perfect world you would replace that beam.  
If you do replace it, you will still want diagonal braces from the vertical log to the horizontal.
If you have the room then diagonals AND a vertical post in the middle would be ultra strong (over kill)

Thank you for your detailed insight Thomas. Your response makes sense, hence I would prefer not to replace it, but instead to overbuild supports because thicker wood is not so easy to come by.

John, do you have any further reasoning to support your reply?

If the pictures reflect where the project stands now, I would replace it rather than to have regrets in the future.  If the project was further along, I would probably leave it in place.  

Can you drill holes horizontally through the log and install some metal plates [ horror!! ] on both sides to reinforce it?

I don't have a lot of experience, but that looks sketchy as hell to me. I'm with John on this one. It's not so hard to replace at this point.

If you are going to use it, I think a bunch of 1/4" bolts with big washers on either end might work better than a steel band to stabilize the split. I'm not sure how you'd get the band good and tight.

What are you going to use for roofing? Even at that angle, wet snow could build up on something like shingles. And wet snow is heeeaavy. Every January, the roof collapses start happening here, and we build for snow.

Oh, sorry, John. I started my reply a while ago, got busy with something else, then just hit submit without seeing your last reply. Steel plates might be better than bolts.

Thank you for the replies. I decided I will bite the bullet and I managed to find a tree with twice the diameter (overkill) that fell this past season.

I ended up replacing it with a hulk.